The invention is concerned with a blocked polyisocyanate having a use in coating composition and in particular a powder coating composition as the cross-linking agent. In particular, the coating composition is comprised of various segments which during the curing of these segments form a flexible adherent film upon the substrate. Most preferably, the powder is comprised of a polymerized acrylic resin and a cross-linking agent having a blocked isocyanate group.
The problems to be solved for prior coating compositions, especially powder paint coating compositions are that they had poor flow during cure and the films had poor gloss. In addition, there were high temperatures required for fusing, while the cross-linking reaction temperature was even higher, about 375.degree.-500.degree. F. Further, blocked isocyanate materials had a tendency to cake in storage at low temperatures, such as, 70.degree.-80.degree. F and that the coatings were brittle, lacked flexibility and adhesion.
During the cure of the coating composition, completely blocked materials (e.g. isophorone diisocyanate blocked with oximes) have a tendency to be low boilers thereby causing boil-off especially at high temperature curing. A satisfactory coating cannot be obtained when such impurities are present in the coating composition. Also, the same materials have a tendency to cause caking during storage because of their low molecular weight.
The solution to these problems would be to have as pure a cross-linking agent as possible, which is compatible with the polymerized resin. In addition, the cross-linking agent should impart flow at a desired low curing temperature as a solvent and act as an external plasticizing agent for the resin, yet be stable below desired storage temperature; it also should have functional groups which will cross-link with the resin at as low a temperature as efficiently possible and after cross-linking will become part of film and act as an internal plasticizer.
U.S. Pat. No. 3,660,143 teaches a radiation curable paint binder having the addition product of a hydroxy functional acrylic rubber particle, a diisocyanate and a hydroxyalkyl acrylate and vinyl monomers.
U.S. Pat. No. 3,542,586 teaches a radiation curable paint with a vinyl binder having pendant monoester groups.
U.S. Pat. No. 3,317,463 teaches the utilization of blocked isocyanate compositions (blocked with hindered phenols) in preparation of urethanes, while U.S. Pat. No. 3,583,943 teaches the blocking of an isocyanate group with a ketoxime.
U.S. Pat. No. 3,621,000 teaches a blocked isocyanate with a bifunctional polyether.
U.S. Pat. No. 3,659,003 teaches a copolymerized acrylic resin which is reacted with a monoblocked diisocyanate to produce molding powders. The difficulty with all of the previous art is that there is not a simple means of incorporating a plasticizing effect into a cross-linked polymerized resin by a convenient processing step in the formation of a resin film as has been described above.
Further the difficulty with all of the previous art is that there is not a simple means of formulating a powder composition which can rapidly be cured to the desired film at a low temperature.